This invention relates to compound engines and more particularly to power plants having a fuel-burning positive displacement engine stage supplemented by an additional engine stage driven by the exhaust gas pressure from the first engine stage.
Single-stage positive displacement engines, including both reciprocating and rotary engines, produce usable power as a result of the expansion of hot pressurized gases against pistons or other movable elements which are coupled to an output shaft. Practical considerations usually make it necessary to release or exhaust such pressurized gases prior to full expansion. Consequently the final phase of gas expansion, following opening of the exhaust valves or the like, does no useful work, giving rise to a form of thermodynamic loss which is often referred to as blowdown loss.
Greater recovery of the energy content of consumed fuel can be realized with compound engines which have a second engine stage, typically a turbine, that is driven by the exhaust gas from a positive displacement first engine stage. The second engine stage produces additional power from the final phase of gas expansion and the additional power is variously delivered to a second independent output shaft or to the crankshaft of the positive displacement engine or, in the case of turbocharged systems, to a compressor which precompresses air at the intake of the positive displacement engine stage to further increase thermodynamic efficiency.
In prior compound engines, blowdown losses continue to be the greatest thermodynamic loss. For example in a turbocharged diesel engine the cylinder pressure at the exhaust valve opening point may typically be 150 psi (1034 kPa). Upon opening of the exhaust valve, this gas pressure blows down or reduces to the exhaust manifold pressure which is typically about 25 to 30 psi (172 to 207 kPa) without doing any useful work. The final phase of gas expansion occurs in the turbine developing additional powder which would otherwise also be lost. Although the sizable blowdown loss has already occurred, the turbine still produces enough energy to operate a compressor which raises the engine inlet manifold pressure to around 30 psi (207 kPa) or somewhat more and this increases the efficiency of the engine thermodynamic cycle by boosting or precompression of intake air. In some compound engines, the turbine generates more power than is needed to operate the compressor and the surplus is transmitted to the positive displacement engine main shaft to supplement the power produced directly by the engine, although this does not usually occur to a significant extent in a typical turbocharged engine.
In some forms of steam engines, as opposed to internal combustion engines, blowdown losses are substantially eliminated. In double or triple expansion steam engines for example the lower pressure stage precompresses a portion of the exhaust steam trapped in the cylinder to a pressure level similar to that of the exhaust passage of the stage ahead of it. A line between the two stages is then opened transferring the charge of steam from one stage to the next without any appreciable blowdown loss. A single-stage steam engine precompresses the steam to boiler pressure prior to opening the inlet valve, avoiding blowdown losses in transit from the boiler to the engine.
There appear to have been few prior attempts to effect a reduction of blowdown losses in internal combustion engines, conceivably because this requires a relatively high exhaust manifold pressure or back pressure as it is commonly termed. A high exhaust manifold pressure or back pressure is universally believed to increase certain thermodynamic losses resulting in a substantial lessening of engine efficiency. An indicator card cycle diagram or pressure-volume diagram for a single-stage internal combustion engine having a high back pressure exhibits a large negative energy loop which would seemingly indicate that such a cycle would not be desirable.
Some attempts to reduce blowdown loss in compound engines of the turbocharged diesel form have been made by providing individual exhaust conduits from each cylinder of the diesel to the intake of the turbine with each such exhaust conduit being of smaller cross-section area than the exhaust valve opening at the associated cylinder. This reduces the pressure drop, and therefore the blowdown losses, to an extent that offsets the loss from the increased back pressure. However, sizable blowdown losses remain, pressure fluctuations in the small conduits are very severe and the turbine operates at widely varying pressures and efficiencies. These effects are not conducive to high efficiency of the overall compound engine operation.
Elimination of blowdown losses in a practical and effective manner would substantially lower the Specific Fuel Consumption factor of a compound engine thereby causing a greater proportion of the chemical energy content of consumed fuel to be delivered to the output shaft in the form of usable kinetic energy.